linux-kernel - Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

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Date:   Fri, 25 Feb 2022 15:51:22 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 18/39] x86/ibt: Add IBT feature, MSR and #CP handling

On Fri, Feb 25, 2022 at 11:51:01AM +0100, Peter Zijlstra wrote:
> > > +bool ibt_selftest(void)
> > > +{
> > > +	unsigned long ret;
> > > +
> > > +	asm ("1: lea 2f(%%rip), %%rax\n\t"
> > > +	     ANNOTATE_RETPOLINE_SAFE
> > > +	     "   jmp *%%rax\n\t"
> > > +	     ASM_REACHABLE
> > > +	     ANNOTATE_NOENDBR
> > > +	     "2: nop\n\t"
> > > +
> > > +	     /* unsigned ibt_selftest_ip = 2b */
> > > +	     ".pushsection .rodata,\"a\"\n\t"
> > > +	     ".align 8\n\t"
> > > +	     ".type ibt_selftest_ip, @object\n\t"
> > > +	     ".size ibt_selftest_ip, 8\n\t"
> > > +	     "ibt_selftest_ip:\n\t"
> > > +	     ".quad 2b\n\t"
> > > +	     ".popsection\n\t"
> > 
> > It's still seems silly to make this variable in asm.
> > 
> > Also .rodata isn't going to work for CPU hotplug.
> 
> It's just the IP, that stays invariant. I'm not sure how else to match
> regs->ip to 2 in #CP.

Ah, I see what you mean now.  Still, it can just reference the code
label itself without having to allocate storage:

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 4806fa0adec7..cfaa05ddd1ec 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -225,7 +225,7 @@ static void handle_endbr(struct pt_regs *regs)
 	BUG();
 }
 
-extern const unsigned long ibt_selftest_ip; /* defined in asm beow */
+void ibt_selftest_ip(void); /* code label defined in asm below */
 
 DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
 {
@@ -237,7 +237,7 @@ DEFINE_IDTENTRY_ERRORCODE(exc_control_protection)
 	if (WARN_ON_ONCE(user_mode(regs) || error_code != 3))
 		return;
 
-	if (unlikely(regs->ip == ibt_selftest_ip)) {
+	if (unlikely(regs->ip == (unsigned long)ibt_selftest_ip)) {
 		regs->ax = 0;
 		return;
 	}
@@ -249,22 +249,12 @@ bool ibt_selftest(void)
 {
 	unsigned long ret;
 
-	asm ("1: lea 2f(%%rip), %%rax\n\t"
+	asm ("1: lea ibt_selftest_ip(%%rip), %%rax\n\t"
 	     ANNOTATE_RETPOLINE_SAFE
 	     "   jmp *%%rax\n\t"
 	     ASM_REACHABLE
 	     ANNOTATE_NOENDBR
-	     "2: nop\n\t"
-
-	     /* unsigned ibt_selftest_ip = 2b */
-	     ".pushsection .rodata,\"a\"\n\t"
-	     ".align 8\n\t"
-	     ".type ibt_selftest_ip, @object\n\t"
-	     ".size ibt_selftest_ip, 8\n\t"
-	     "ibt_selftest_ip:\n\t"
-	     ".quad 2b\n\t"
-	     ".popsection\n\t"
-
+	     "ibt_selftest_ip: nop\n\t"
 	     : "=a" (ret) : : "memory");
 
 	return !ret;